Information concerning the development and properties of the biosynthetic pathways of surfactant lecithin has led to a further understanding of the patho-physiology of the respiratory distress syndrome (RDS). However, measurements of the source of alveolar surface active lecithin and its rate of appearance and disappearance at birth have not been made. The purpose of the project is, by using a rabbit model system, to study the kinetics of appearance and the source (de novo synthesis vs. storage) of the surface active alveolar lecithin appearing at birth. Further, the effect on alveolar surfactant lecithin of any phospholipase activity found in alveolar wash will be measured. The study will be extended to clinical material, with neonatal tracheal secretions being used as a source of phospholipase activity. These enzymatic activities will be correlated with the clinical status of the neonate.